A common reaction for forming the octachlorodirhenate(III) anion is to heat tert-n-butylammonium perrhenate in benzoyl chloride at 209 ºC. If you look up the info for benzoyl chloride, one sees that the boiling point is only 197 ºC. To increase the boiling point, publications suggest closing off the system with a mercury bubbler, which increases the pressure enough to bump the boiling point of the liquid to 209 ºC.

In the teaching lab, use of mercury is highly frowned upon… are there any reasonable alternatives to a mercury bubbler to increase the boiling point (and thus, the reaction temperature) for this reaction? Publications report a 30-40% decrease in yield if the reaction temperature is lowered to 196 ºC, so there's a lot of motivation to keep the temperature up.

  • $\begingroup$ Silicone oil bath it is then I guess $\endgroup$
    – Mithoron
    May 3 '17 at 17:43
  • $\begingroup$ Can you just use a sealed tube? $\endgroup$
    – jerepierre
    May 3 '17 at 20:03
  • $\begingroup$ According to Wikipedia: "A gas bubbler is a piece of laboratory glassware which consists of a glass bulb filled with a small amount of fluid — usually mineral or silicone oil, less commonly mercury." I am not particularly familiar with this procedure and may be missing something basic, but it seems like a mineral or silicone oil would be the more common way to go (as Mithron also said). $\endgroup$
    – airhuff
    May 3 '17 at 20:34
  • $\begingroup$ Silicone oil and mineral oil don't provide the weight (literally, weight of liquid providing the over-pressure) needed to raise the boiling point high enough. I suppose that a bubbler of sufficient length could be made to make the weight of oil equal the weight of mercury in a standard-size bubbler. $\endgroup$ May 3 '17 at 21:42

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.